1. Field of Use
This invention relates generally to apparatus and methods for register control in web processing apparatus, such as multi-deck rotogravure color printing presses, or the like, which employ adjustably positionable web or cylinder compensator mechanisms for varying web length between printing decks to eliminate register errors.
2. Description of the Prior Art
Some prior art register control systems sense and measure only the magnitude and direction of register errors and adjust the compensator mechanism position accordingly to effect a change in web length and thereby eliminate register error. In such prior art systems, compensation is relatively slow and allows considerable register error to accumulate while a correction is being made. Attempts to increase compensator rate of change in such systems result in instability, oscillation and poor registration. In effect, such prior art systems involve random (trial and error) positioning of the compensator until an optimum position of the compensator is found wherein register error is eliminated, but due to the cyclic nature of the process, this optimum position is rarely achieved. One type of prior art register control system employs a strain gauge for measuring web tension between successive roller nips and operates to locate the compensator in a position wherein a desired web tension, considered to be indicative of no register error, is maintained. However, there are many variables which affect web characteristics and web tension, such as moisture and humidity, and therefore the last-mentioned prior art systems allow register errors to occur, even though a predetermined web tension is being maintained, because web conditions are actually changing.